Microtubules are composed of .alpha./.beta.-tubulin heterodimers and constitute a crucial component of the cell cytoskeleton. Furthermore, microtubules play a pivotal role during cell division, in particular when the replicated chromosomes are separated during mitosis. Interference with the ability to form microtubules from .alpha./.beta.-tubulin heterodimeric subunits generally leads to cell cycle arrest. This event can, in certain cases, induce programmed cell death. Thus, natural products and organic compounds that interfere with microtubule formation have been used successfully as chemotherapeutic agents in the treatment of various human cancers.
Pentafluorophenylsulfonamidobenzenes and related sulfhydryl and disulfide modifying agents (see, e.g., compound 1; 2-fluoro-1-methoxy-4-pentafluorophenylsulfonamidobenzene; FIG. 1C) prevent microtubule formation by selectively covalently modifying .beta.-tubulin. For example, compound 1 does not covalently modify all of the five known .beta.-tubulin isotypes. Instead, binding is restricted to those .beta.-tubulin isotypes that have a cysteine residue at amino acid position 239 in .beta.-tubulin. Such isotypes include .beta.1, .beta.2 and .beta.4-tubulin. The other two isotypes (.beta.3 and .beta.5) have a serine residue at this particular position (Shan et al., Proc. Nat'l Acad. Sci USA 96:5686-5691 (1999)). It is notable that no other cellular proteins are modified by compound 1.
Although .beta.-tubulin modification can be monitored using the tritiated derivative of compound 1 or other .beta.-tubulin modifying agents, in many cases this method is either impractical or not sensitive enough. Specific, sensitive methods of detecting .beta.-tubulin modification are needed for diagnostic applications and for dose monitoring in patients receiving chemotherapeutic agents that modify .beta.-tubulin.
The present application is related to U.S. Pat. No. 5,880,151, issued Mar. 9, 1999; PCT 97/02926, filed Feb. 22, 1997; PCT 97/12720, filed Jul. 18, 1997; PCT 98/16781, filed Aug. 13, 1998; PCT 99/13759, filed Jun. 16, 1999; and PCT 99/16032, filed Jul. 15, 1999, herein each incorporated by reference in their entirety.